1. Field of the Invention
The present invention relates to technology of electrical superconductors. The importance of materials which have superconducting properties has greatly increased very recently. The discovery of new superconducting materials, in particular of the rare-earth/Ba/Cu/O type, led to a considerable expansion of the possible applications of superconductors, since these materials already become superconducting at temperatures above 50 K.
The invention relates to the provision of suitable starting materials for the further development and improvement of components made of a ceramic high temperature superconductor in wire form, wherein the requirements of industrial large-scale production are to be taken into account.
It relates in particular to a process for the production of an elongate body consisting of longitudinally aligned acicular crystals of a superconducting ceramic material based on REBa.sub.2 Cu.sub.3 O.sub.6.5+y, where RE means a rare earth metal and 0&lt;y&lt;1, and the superconducting single crystals are produced by sintering.
2. Discussion of Background
It has been shown that ceramic high temperature superconductors of the class REBa.sub.2 Cu.sub.3 O.sub.6.5+y (RE=rare earth metal, 0&lt;y&lt;1) have strongly anisotropic properties in every respect. This depends on the crystal structure (perovskite lattice) and is particularly the case for the critical current density j.sub.c (current carrying capacity). The grain boundaries in polycrystalline ceramics limit the critical current densities to small values, so that these materials are not suitable for most technical applications at the boiling temperature of liquid nitrogen (T=77 K.). For sintered samples of polycrystalline YBa.sub.2 cu.sub.3 O.sub..about.7, critical current densities j.sub.c up to a maximum of 100 A/cm.sup.2 were measured up to now in magnetic fields &gt;1 T. However, applications in, e.g., magnet coils require current densities which are greater by a factor of about 1,000.
It is apparent from the above that an oriented crystal structure can be expected to provide substantially higher critical current densities than crystallites arranged in a chaotic and statistically arbitrary manner. The highest current densities j.sub.c, of more than 10.sup.5 A/cm.sup.2, were observed in thin superconducting layers grown epitaxially on SrTiO.sub.3 single crystals. However, this process necessitates expensive single crystals as substrates and cannot be applied to the production of very long wires (multiple filaments) and strips.
It is known that particles of the compound YBa.sub.2 Cu.sub.3 O.sub..about.7 can have an acicular or lamellar particle morphology arising from growth anisotropies. The short axis of these particles then corresponds crystallographically to the c-axis of the lattice. The long axes of the particles reproduce the a- or b-axes of the lattice, wherein these directions can accept markedly higher critical currents than the c-axis.
An effect known as secondary recrystallization is known in the literature. In secondary recrystallization, grain growth in a structure occurs in only a very small number of grains, which act as nuclei. The remaining grains in the structure hardly change, until they are completely used up by the growth of the nuclei. The nuclei can grow to grains of up to 1 mm. If the nuclei have a preferred orientation before the beginning of grain growth, a structure with a corresponding texture results from secondary recrystallization.
The following literature is cited in relation to the state of the art:
Quentin Robinson, P. Georgopoulos, D. Lynn Johnson, Henry O. Marcy, Carl R. Kannewurf, S.-J. Hwu, Tobin J. Marks, K. R. Poeppelmeier, S. N. Song, J. B. Ketterson, "Sinter-Forged YBa.sub.2 Cu.sub.3 O.sub.7-.delta. ", Materials Research Center, Northwestern University, Evanston, Ill. 60201, Advanced Ceramic Materials, Vol. 2, No. 3B, Special Issue, June 1987 PA1 R. W. McCallum, J. D. Verhoeven, M. A. Noack, E. D. Gibson, F. C. Laabs and D. K. Finnemore, "Problems in the Production of YBa.sub.2 Cu.sub.3 O.sub.x Superconducting Wire", Ames Laboratory, USDOE, Iowa State University, Ames, Iowa 50011, Advanced Ceramic Materials, Vol. 2, No. 3B, Special Issue, June 1987 PA1 G. S. Grader, H. M. O'Bryan and W. W. Rhodes, "Improved Press Forging of Ba.sub.2 YCu.sub.3 O.sub.x Superconductor", 1988, American Institute of Physics, Appl. Phys. Lett. 52(21), May 23, 1988, pp. 1,831-1,833.